1. Field of the Invention
The present invention relates to a voltage generating device which generates a voltage which does not depending upon the temperature.
2. Description of the Prior Art
Such type of prior art voltage generating devices comprises a voltage source including a semiconductor PN junction for generating a voltage which negatively changes with temperature and a voltage source for generating a thermal voltage (kT/q) which positively changes with temperature, both voltage sources being connected in series for cancelling the changes in voltage with temperature with each other.
The structure of a prior art voltage generating device is shown in FIG. 1. In FIG. 1, a reference numeral 1 denotes an output terminal of a voltage generating device; 21 denotes a current source; 22 a resistor; 23 a diode-like connected transistor. A voltage on the output terminal 1 is obtained by causing a current to flow from the current source 21 through the series-connected resistor 22 and diode 23. The current source, 21 is a band gap current source as disclosed in JP-A-60-191508. The current value Ics is determined by equation (1). EQU Ics=(k.times.T/q).times.ln(N).times.Rcs (1)
wherein k denotes the Boltzmann's constant; T denotes an absolute temperature; q denotes the charge of electrons; N denotes a constant; Rcs denotes a current presetting resistance.
The voltage Vo on the output terminal 1 can be expressed by equation (2). EQU Vo=Vf23+R22.times.Ics (2)
wherein Vf23 and R22 denote the forward voltage of the transistor 23 and the resistance of the resistor 22, respectively.
The first clause in equation (2) denotes the forward voltage of the diode-like connected transistor 23. It is generally well known that this voltage changes at -2 mV/deg with temperature when it is about 650 mV. Therefore, a change in voltage with temperature in the second clause is preset to a value which has the opposite sign, and is equal to the absolute value of that in the first clause thus, the changes in voltage with temperature in the first and second clauses can be cancelled with each other. Briefly, in order to make Vo a temperature independent voltage, equation (2) is put into the second clause to provide equation (3). ##EQU1## A change in voltage with temperature is obtained by differentiating equation (3) with respect to the absolute temperature T. If the change is represented by +2 mV, equation (4) is obtained. ##EQU2##
By putting equation (4) into equation 3 and by setting the thermal coefficients of R22 and Rcs equal to each other and T=300 K, equation (5) is obtained. ##EQU3##
Accordingly, if R22 or Ics is preset in such a manner that R22.times.Ics=600 mV, Vo is determined as about 1.25 V in accordance with equation (2). Vo is independent of temperature. This approach has been widely adopted since the thermal coefficients of R22 and Rcs can be easily made equal if these components are formed on a single semiconductor chip.
In such a manner, even the prior art voltage generating device is capable of generating a voltage which is independent of temperature.
However, the prior art voltage generating device can not be used for a circuit which requires a power source voltage which is lower than 1.25 V since the voltage which is independent of temperature only down to voltage of 1.25 V. In other words, since the first clause in equation (2) is fixed as 650 mV, the second clause should be equal or lower than 600 mV. Accordingly, Vo is dependent upon temperature for values below 1.25 V.